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6 Answers

Alternator output

Asked by: 2449 views Aircraft Systems, Flight Instructor, General Aviation

Why is the alternator output higher than the battery output? For example, a PA-44 alternator provides 60-70 amps while the battery is only 35 amps.

Thank you.


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6 Answers

  1. Nibake on Sep 30, 2014

    Can you clarify what you mean by the battery being 35amps? Batteries are normally rated for a certain number of amp hours. To say a battery is “35 amps” is meaningless without additional information. Sometimes a number will also be given for cranking amps, but 35 is way too low so I don’t think that is the number you are referring to. My guess would be that you have a 35amp hour battery, which means it could put out 35amps for 1 hour at 12v (assuming you have a 12v system) when fully charged before completely discharging.

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  2. Nibake on Sep 30, 2014

    *Correction – that would be 35amps over one hour at 10v (minimum of 1.67v per cell for a 12v battery)

    On the other hand, your alternator may very well be specifying the peak amps it is rated for at any given time, in which case your alternator must put out more amps than the maximum possible load you can put on it. If you can load your alternator to more than 80% then your aircraft won’t pass its next inspection without having (a) placards or monitoring devices installed, (b) a bigger alternator put in, or (c) the electrical load reduced.

    This is not related to the amp-hour rating of your battery. Hopefully it all makes sense, if not let me know and maybe I can explain more in depth.

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  3. Oren on Sep 30, 2014

    So alternator is apms and battery is amp/hour which are totally different things?

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  4. Nibake on Sep 30, 2014

    That is correct, they are different things. Probably the cause of your confusion.

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  5. Oren on Oct 01, 2014

    Sounds like you have in-depth knowledge of electrical systems. Would you mind explaining? Let’s take the PA-44 example – Alternator 14V and 60amps, and battery 12V 35amp/hour. How does it work? The alternator provides 60amps to all the electrical devices and charges the battery? Isn’t it a problem that the alternator provides 14V to a 12V battery (overvoltage)?

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  6. Nibake on Oct 01, 2014

    14V isn’t really overvoltage for a battery, it’s actually just right. If you think of voltage as potential difference or pressure, a 12v alternator wouldn’t charge a 12v battery because the pressure would be the same on both ends and no current would flow. However, a 14v alternator has greater pressure and current will flow to the battery. If, say, you put 28v to a 12v battery, then yes, that would be overvoltage and things would start to get…hot.

    A battery’s life in an airplane is pretty relaxed when everything is working as it should. It just starts the engine, and then sits back and gets a little charge put back into it while the alternator does all the work.

    “The alternator provides 60amps to all the electrical devices and charges the battery?”

    Alternator output is controlled so that you aren’t putting out more power than you need to. It shouldn’t be cranking out 60 amps all the time. The alternator is supply a very low amount of power which determines how much power it will put out. I don’t really know what the ratio is but it is designed so that if you need 40 amps, say, maybe 100 milliamps will be used to excite the alternator and output the required amps. If we need 50 amps, then maybe 120 milliamps will be put in. (don’t quote me on that, just trying to give you the general idea.) If you are flying along in the daytime with just your radios on and all your lights off it might be putting out only 5-10 amps out of its maximum of 60.

    Also, I may have been mistaken in my previous answer about the battery being rated for one hour, and A&P IA told me that the rating is normally for 5 hours, but I don’t know where the source for that is.

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